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Slideshow: Sperm Whales Adopt Deformed Dolphin

Traveling. An unusual, mixed-species group consisting of sperm whales and a single bottlenose dolphin with a spinal malformation swim together in the Azorean archipelago.

Credit: Alexander D. M. Wilson/Aquatic Mammals

Milling. While the group is socializing near the surface, the dolphin rubs its body in a friendly way against one of the whales. The whales sometimes rub back.

Credit: Alexander D. M. Wilson/Aquatic Mammals

Nuzzle. The dolphin nuzzles one of the whale calves with its rostrum (snout).

Credit: Alexander D. M. Wilson/Aquatic Mammals

Mouth. The dolphin positions himself just in front of the open jaws of an adult female sperm whale. Calves and subadults do this, too, but the reason is unknown.

Credit: Alexander D. M. Wilson/Aquatic Mammals

Contact. The dolphin touches its pelvic region to the lower jaw of the adult, a gesture that might strengthen social bonds between animals.

Sperm whales are fierce squid hunters, but they also have a softer side. In a serendipitous sighting in the North Atlantic, researchers have discovered a group of the cetaceans that seem to have taken in an adult bottlenose dolphin with a spinal malformation, at least temporarily. It may be that both species simply liked the social contact.

Creatures form "friendly" connections with members of other species throughout the animal kingdom. These often short-lived relationships can offer increased protection from predators and more effective foraging. Some particularly unusual alliances illustrate that they can also satisfy a social craving. For example, the signing gorilla Koko had a pet cat named All Ball; in a Kenyan nature park, a hippopotamus, Owen, grew close to a giant tortoise, Mzee.

Among ocean-dwelling mammals, dolphins are perhaps the most gregarious. They've been spotted traveling, foraging, and playing with a wide variety of other animals, including many whales. On the other hand, as far as the authors of the forthcoming paper in Aquatic Mammals know, sperm whales had never been reported cozying up to another species. Specialized deep-water hunters who travel great distances, the whales are more timid than dolphins and harder for people to observe.

Indeed, behavioral ecologists Alexander Wilson and Jens Krause of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin did not expect to find a mixed-species group when they set out to observe sperm whales (Physeter macrocephalus) some 15 to 20 kilometers off the island of Pico in the Azores in 2011. But when they got there, they found not only a group that included several whale calves, but also an adult male bottlenose dolphin ( Tursiops truncatus). Over the next 8 days, they observed the dolphin six more times while it nuzzled and rubbed members of the group (see slideshow). The sperm whales seemed to at least tolerate it; at times, they reciprocated. "It really looked like they had accepted the dolphin for whatever reason," says Wilson, who was snorkeling nearby. "They were being very sociable."

The researchers could be sure that the bottlenose dolphin was the same one each time because it had a rare spinal curvature that gave its back half an "S" shape. Although the dolphin seemed otherwise healthy, that probable birth defect could be the key to understanding its attachment to the sperm whale group. Very few predators stalk the Azorean waters, so they doubt that it needed the whales for protection. But they speculate that the malformation could have put the animal at a disadvantage among its own kind. Perhaps it couldn't keep up with the other dolphins or had a low social status.

"Sometimes some individuals can be picked on," Wilson says. "It might be that this individual didn't fit in, so to speak, with its original group." The dolphin was able to stay with the whales because they swim more slowly and always leave a "babysitter" near the surface with the calves while the other adults dive deep.

Less clear is what was in it for the sperm whales. This study shows that they have a capacity for these types of relationships, which implies that they may sometimes get benefits from them, Wilson says. However, there's no obvious advantage on their side in this case. What's more, cetacean ecologist Mónica Almeida e Silva of the University of the Azores in Portugal, who was not involved in the study, says that sperm whales have good reasons not to like bottlenose dolphins, which she has often seen chasing and harassing whales and their calves. "Why would sperm whales accept this animal in their group?" she says. "It's really puzzling to me."

Nonetheless, we shouldn't be tempted to "overread" the whales' motivations as pity for the dolphin, says behavioral biologist Luke Rendell of the University of St. Andrews in the United Kingdom. Interpreting is hard given the observation's briefness and rarity, as well as how little is known about these particular whales. They might simply enjoy the dolphin's attentions, or "they could just be thinking, 'Wow, this is a kind of weird calf.' "